Vinyl alcohol polymers (vinyl alcohol polymer may be hereinafter abbreviated as PVA) are one of the few crystalline water-soluble polymers, and have excellent interface properties and strength properties. For this reason, vinyl alcohol polymers are used in paper processing, fiber processing, and emulsion stabilizers, and also occupy an important place as PVA films, PVA fibers, etc. In addition, functional improvement in which specific performances are enhanced by control of the crystallinity or introduction of a functional group has been pursued, and various types of so-called modified PVAs have been developed.
Mercapto groups are highly reactive functional groups, and introduction of mercapto groups into PVAs is beneficial because the introduction allows, for example, production of functional polymers such as graft polymers and block polymers, modification of polymers by various polymer reactions, and impartation of water resistance by reaction with a crosslinking agent.
Some modified PVAs having mercapto groups have been hitherto proposed (Patent Literature 1 to 3). However, the mercapto groups in the modified PVAs are introduced only at the molecular chain terminals, which poses a problem in that the number of reaction points is small and sufficient water resistance is not exhibited by reaction with a crosslinking agent.
Carboxyl-modified PVAs and acetoacetic ester group-containing PVAs are known as PVAs whose reactivity with crosslinking agents has been made use of, and are used, for example, in coating agents for forming printing paper or coat layers (Patent Literature 4 and 5). However, carboxyl group-modified PVAs do not have sufficient reactivity and, therefore, a curing step including storage at 30 to 40° C. for about 1 day to 1 week is required in order to achieve a desired level of water resistance. Acetoacetic ester group-containing PVAs have excellent reactivity, and thus allow impartation of water resistance. However, there is a problem in that reaction between acetoacetic ester groups introduced in side chains and a crosslinking agent rapidly proceeds, leading to poor viscosity stability and short usable life of the resulting solution.